The UDTeams performed very hazardous tasks in the waters of the invasion beaches many days BEFORE an invasion. they would map approach gradiants, water depths at different tides, locate obsticles & prepare them for demolition just prior to the landing & recon enemy beach defences.
UDT - 3 "SEABEE DEMOLITION TEAM" Welcoming in the Marines at Guam. Look at the date on the sign. 7-14-44. The invasion of Guam started on 7-21-44 ! They were there seven days before the invasion.
As far as the uniforms, for invasions and operations, the Seabees always wore the same battle dress as the marines which caused a lot of the problems we encounter now in researching the Seabees. To the combat correspondents & reporters they were indistinctable from the marines and identified as marines in films and photos. Even Orders of Battle and After Action Reports would list them as Marine Battalions.